In biology and analytical chemistry, microarrays are widely used in detecting biological macromolecules such as DNA, proteins, antibodies, glucoconjugates and cancer markers. A common approach in detecting macromolecules is the lock-key chemistry where a nano-liter drop of macromolecular soup including target “key” molecule interacts with lock molecules spotted on a substrate. “Lock” molecules selectively bind to the “key” molecules of interest, producing a fluorescence signal.
Spotting and evaporation of drops are essential processes for production of efficient, high quality microarrays. However, the efficiency of aforementioned methods is hampered by so called coffee ring/stain effect. Coffee stain effect is a generic physical phenomenon that occurs whenever a suspension (e.g. a drop consisting of a volatile solvent and a nonvolatile solute) on a surface evaporates on a solid surface (“Capillary flow as the cause of ring stains from dried liquid drops”; Deegan et al., Jul. 15, 1997). The contact line pinning leads to enhanced evaporation in the vicinity the contact line and leaves behind a heterogeneous ring like structure called coffee stain. Quality of spotting is crucial for expanding microarray applications to other fields such as gene regulatory networks, protein-protein interactions where precision and number of samples to be analyzed is more demanding.
For example, a coffee stain effect can be observed in the figures of the publication “A micro particle sampler using electrowetting-actuated droplet sweeping”, Zhao et al., pages 129-134, Transducers'05, The 13th International Conference on Solid-State sensors, Actuator and Microsystems, Seoul Korea, Jun. 5-9, 2005. In that publication, droplets are moved over a hydrophobic surface, before their evaporation, for collecting particles therefrom (for the purpose of sampling). According to the publication, a top electrode in the form of a glass plate having a grounding ITO layer—used for transporting the droplets—is removed to enhance the evaporation. The evaporation was only performed to demonstrate that the particles are indeed successfully swept away from the superhydrophobic surface for the purpose of sampling.
Evaporation of droplets can be achieved in various ways. For example, the publication “An EWOD droplet microfluidic chip with integrated local temperature control for multiplex proteomics”, Nelson et al., MEMS 2009, MEMS 2009, IEEE 22nd international conference, Piscataway USA, 20 Jan. 2009, pages 280-283, discloses pinning samples utilizing hydrophilic patches, and heating the pinned samples for their evaporation. In this publication, heating is performed by resistive heating. Electrowetting is only used to move the drop onto the resistive plate.
Various methods are known to improve spatial stain homogeneity, as is described in US2007/0170058 (Lee et al.). US'058 propose a device that controls particle distribution using electroosmotic flow at the bottom of the droplet. To that aim, the device comprises a substrate having two electrodes. One of the electrodes is ring-shaped and must extend along the rim of the droplet during operation. The second electrode must be located at the center of the droplet. During operation, DC (direct current) electric field is applied to a droplet using both electrodes, causing radial electroosmotic flow in a bottom part of the droplet. Application of high voltages is avoided since such voltages hampers uniform distribution, according to US'058. A uniform solute deposition can only be obtained when the electric field has a specific strength.
One disadvantage of this method is that the droplet must be accurately positioned with respect to both electrodes. Also, as follows from the experiments mentioned in the publication, setting electrode voltages to the appropriate values for achieving the desired radial flow is relatively cumbersome, and is highly dependent on the electrical characteristics of the constituents of the droplet. Besides, the electroosmotic flow based method relies on direct electric contact with liquid of interest. This poses undesired issues related to bio compatibility. Furthermore, efficiency of such a method strongly relies on properties of solute which varies dramatically for biomolecules. In addition to this, the electrodes are not flexible for miniaturization as microarrays may require hundred thousand spots on a chip. Thus, this method does not provide a “flexible and solid” solution for coffee stain phenomena in bio-microarrays.
The present invention aims to provide an improved method for treating one or more drops of liquid. Particularly, the invention aims to provide a commercially viable method, wherein suppression or reduction of the ring stain effect can be achieved in an efficient, easily controllable manner.